As semiconductor technology progresses, shrinking device dimensions has become an increasingly complex task. Complementing metrology tools, allowing similar improvements in measurement capabilities, are critical for the continual process of this development.
Optical metrology can acquire highly accurate and precise information on the geometry and material properties characterizing patterned structures with small dimensions of the pattern features (critical dimensions). Several physical quantities are commonly measured by optical metrology and in particular by optical critical dimensions (OCD) techniques. For example, optical reflectometry measures the reflection intensity for a broad spectrum, over a single (or small set) of incidence directions and different polarizations. Ellipsometry allows, in addition, access to information on the relative phase between different polarization states.
Another important attribute of light scattered from a patterned structure is its phase, namely a relative phase between incident and reflected light beams. This phase can be measured using a variety of interferometry techniques. These methods are based on separating the light beam into two parts so that only one part interacts with (is reflected from) the sample. The reflected light is then re-interfered with the second part of the beam (“reference beam”), which did not interact with the sample, and a difference in the optical path length traversed by these two parts is accurately controlled. The interference pattern formed by interference of these two light components in a detection plane is then used to extract the spectral phase.
Existing approaches for measuring phase, including interferometry, are highly delicate, require special measurement apparatus with a reference, and are highly sensitive to environment (e.g. system vibrations). Consequently, such methods are not regularly used for in-line OCD metrology, whereas the more robust methods of reflectometry and ellipsometry are customary.